1. Field of the Invention
The present invention relates to an optical fiber component advantageously applicable to the passive alignment technology.
2. Description of the Related Art
Recently, many optical components based on the PLC (Planar Lightwave Circuit) technology are widely used such as an AWG (Arrayed Waveguide Grating), a VOA (Variable Optical Attenuator), and an optical splitter. Those optical components, each with an interferometer or an optical branching unit within an optical circuit, can be combined to configure an optical filter or an optical attenuator. Thus, those optical components are important as key devices in an optical communication system.
To connect a PLC-based optical component to an optical fiber, the optical fiber is usually connected directly to the optical component with an adhesive. An appropriate adhesive, if used for this connection structure, would ensure good characteristics of connection between the optical component and the optical fiber and therefore gives a practical reliability. In a waveguide type optical device configured by combining an optical component and an optical fiber described above, the optical fiber is attached directly to the optical component. Therefore, this structure ensures reliable operation, because there is no air propagation between them nor are the optical characteristics changed by a shift in the optical axis between them.
The PLC-based waveguide type optical device described above, which is highly reliable and operates stably, requires precise optical coupling between the optical waveguide of an optical component and the optical fiber to a precision of sub-μm. The general method for this optical coupling is as follows. That is, the positions of the optical waveguide of the optical component and the optical fiber are optimally adjusted with an optical jig so that the optical coupling loss is minimized and, while holding them in that state, the optical waveguide of the optical component and the optical fiber are fixed with an optical adhesive and the like. Therefore, the task of coupling between the optical waveguide of the optical component and the optical fiber is complex, takes long, and increases the cost.
To solve this problem, the study is now under way for passive alignment that couples an optical fiber to an optical waveguide using a V groove with no need for optical axis alignment.
FIG. 1 and FIG. 2 show an optical waveguide module (hereinafter called a “first prior art”) disclosed in Japanese Patent Application Laid-open No. 2003-227962.
As shown in FIG. 1 and FIG. 2, this optical waveguide module 80 is configured as an optical splitter comprising a V-groove substrate 81, an optical waveguide substrate 86, and a multi-core tape fiber 91. As shown in FIG. 2(a) and FIG. 2(b), the optical waveguide substrate 86 has a single-core waveguide 88a at one end with a plurality of branching waveguides 88b branching from the single-core waveguide 88a. The optical waveguide substrate 86 has alignment marks 90 in the four corners, one for each. The end face 87 of the optical waveguide substrate 86, the end face of the single-core waveguide 88a, an end face 89, and the end faces of the plurality of branching waveguides 88b are all cut obliquely.
The V-groove substrate 81 has a concave portion 83 thereon for housing the optical waveguide substrate 86 and, across the concave portion 83, one V-groove 82 and a plurality of V-grooves 84 are formed on either side. The V-groove substrate 81 has alignment marks 85 for alignment with the optical waveguide substrate 86.
The multi-core tape fiber 91 has the number of optical fibers 92 corresponding to the number of branching waveguides 88b of the optical waveguide substrate 86, and the end faces of the optical fibers 92 are cut obliquely.
The optical waveguide module 80 is assembled as follows. First, the optical waveguide substrate 86 is glued and fixed to the concave portion 83 of the V-groove substrate 81 using the alignment marks 85 and 90. With an optical fiber (not shown) installed in the V-groove 82 of the V-groove substrate 81, the core of this optical fiber is aligned with the core of the single-core waveguide 88a of the optical waveguide substrate 86. At the same time, with the oblique end face of the optical fiber not shown pushed against the oblique end face of the single-core waveguide 88a, the optical fiber not shown is coupled to the single-core waveguide 88a and the optical fiber not shown is held in the V-groove 82 of the V-groove substrate 81.
Similarly, with the optical fibers 92 of the multi-core tape fiber 91 installed in the V-grooves 84 of the V-groove substrate 81, the cores of the optical fibers 92 are aligned with the cores of the branching waveguides 88b of the optical waveguide substrate 86. At the same time, with oblique end faces 93 of the optical fibers 92 pushed against the oblique end faces of the branching waveguides 88b, the optical fibers 92 are coupled to the branching waveguides 88b and the optical fibers 92 are held in the V-grooves 84 of the V-groove substrate 81.
In the optical waveguide module 80 shown in FIG. 1 and FIG. 2, the optical fibers 92 are placed in the V-grooves 84 for passive alignment, and the oblique end faces 93 of the optical fibers 92 are connected to the oblique end face 89 of the optical waveguide substrate 86 to reduce the return loss.
FIG. 3 shows an optical waveguide module (hereinafter called a second prior art) disclosed in Japanese Patent Application Laid-open No. 8-313756.
As shown in FIG. 3, this optical waveguide module 100 comprises an optical waveguide unit 100B on a silicon substrate 101 and an optical fiber alignment unit 100A adjacent to the optical waveguide unit 100B. The optical waveguide unit 100B comprises a cladding 102 and cores 103 formed on the silicon substrate 101. The optical fiber alignment unit 100A has V-grooves 104 for fixing optical fibers, and the V-grooves 104 are formed on the silicon substrate 101 corresponding to the cores 103. The V-grooves 104 are formed on a part of the silicon substrate 101 through anisotropic etching. On the silicon substrate 101, a machined groove 107 is provided between the optical fiber alignment unit 100A and the optical waveguide unit 100B to physically separate them.
The optical waveguide module 100 shown in FIG. 3 is assembled by placing optical fibers 105 in the V-grooves 104, fixing the optical fibers 105 with a glass pressure plate 106 from above, and connecting the optical fibers 105 to the cores 103. An optical waveguide module similar to the one in the second prior art is disclosed also in Japanese Patent Application Laid-open No. 2001-350051.
However, the first prior art has the following problems.
(1) The oblique end faces 93 of the optical fibers 92, which are completely free end faces, sometimes oscillate in the V-grooves 84. The oscillation of the optical fibers 92 in the V-grooves 84 decreases the precision of alignment between the optical waveguides 88b and the optical fibers 92. In addition, if the oblique end faces 93 of the optical fibers 92 are free, the optical fibers 92 are easily broken if an unexpected force is applied to the optical fibers 92 during installation. Those problems are generated more often as the number of optical fiber cores increases.
(2) The need to align the optical waveguide substrate 86 with the V-groove substrate 81 using the alignment marks 85 and 90 complicates the manufacturing process and decreases alignment precision. The alignment, especially the height alignment between the V-groove substrate 81 and the optical waveguide substrate 86, must be precise to 1 μm or lower. However, even if UV photo-curing resin or heat-hardening resin is used for an adhesive for adhering the V-groove substrate 81 to the optical waveguide substrate 86, the height precision is affected by an order of several μm because the resin is hardened and cured. Therefore, as long as an adhesive is used for coupling the substrates 81 and 86, the alignment of the cores of the optical fibers 92 with the cores of the optical waveguides 88 is extremely difficult.
On the other hand, although the second prior art solves the problems described in (2) above, the problem (1) remains unresolved. That is, forming the optical fiber alignment unit 100A and the optical waveguide unit 100B on the same silicon substrate 101 eliminates the need for alignment between the optical fiber alignment unit 100A and the optical waveguide unit 100B. However, because the end faces of the optical fibers 105 are completely free, the end faces of the optical fibers 105 oscillate in the V-grooves 104 and therefore the alignment precision is decreased. In addition, when the end faces of the optical fibers 105 are free, the optical fibers 105 are easily broken when an unexpected force is applied to them during installation.